The technical challenge in the application of autonomous transport systems arises from the specifical conditions at the customers’ sites. Without detailed specifications, no efficient material flow system with driverless transport vehicles can be realized.
Not only the vehicles must be individually designed, because loads, transfer heights and existing conveyor technology are never the same, sometimes not even within one company. Also the communication of the vehicles with the production machines as well as the fleet control system with the existing production control must be integrated exactly. This is the only way to generate transport orders from the material requirements, to map the production process and to comply with all safety requirements.
Therefore, a project with autonomous transport vehicles usually starts with the creation of a specification sheet. For this purpose, all environmental conditions must be analyzed in close cooperation with the customer and the system requirements must be determined. A proper specification is the prerequisite for the successful implementation of a material flow system with driverless transport vehicles.
Specification of a driverless transport system
The following conditions determine how large the vehicle fleet must be in order to achieve enough availability for average production capacity but also be able to cover production peaks:
- Production quantities and speed, type of goods transported
- Production environment: length of transport routes, number of transports per hour per route
- Possible maximum speed: Dependent on aisle widths, frequency of crossing passenger traffic
- Charging stations: Routes, ratio loading time to transport time
- Order management: proportion of empty runs
This detailed analysis has to be carried out both for average productivity and for expected production peaks up to 100 % productivity.
The result is the number of vehicles and loading stations required for both production loads, from which you can estimate the size of the fleet.
The capacity analysis is the basis for further project planning of the transport system and is therefore of utmost importance. It requires a precise understanding of the production processes by the planner and is developed in close cooperation with the customer.
When the required number of charging stations has been calculated in the capacity analysis, the location of every station must be set in the layout. As described under battery technology, the vehicles can be continuously recharged during operation without negative effects on the capacity and lifetime of the battery, and even for better lifetime. Here we are talking about possible short charging times of about 40-60 s when the vehicle is standing at the transfer stations. We call this opportunity charging.
Separate charging stations are also possible, to which the fleet management sends the vehicles if they do not have any transport orders to carry out or if the battery management reports it as necessary.
The vehicle has charging contact surfaces that are spring-loaded at the charging station. The charging current only flows when the vehicle is docked and the contact to the charging station is confirmed by the hardware. As soon as the battery management informs the vehicle that the battery is fully charged or too warm, the charging process is terminated.
The required battery capacity for average and full 100% productivity must also be considered when planning the charging stations and positioning them in the layout.
Interface to machines or loadports
The vehicles must communicate directly with the machines or the loadports in front of them.
To ensure that the load can be transferred safely and does not fall down, it must be ensured that
- the vehicle is correctly positioned (see Navigation)
- both sides are really ready to hand over/take over
- a total emergency stop is possible (for example, if an operator sees that the load transfer fails and presses the emergency stop button on the vehicle, the conveyor system on the machine must also stop)
There are many possibilities to implement a hardware handshake procedure between the vehicles and the transfer points.
Measurements of the WLAN coverage on the production area are necessary to determine the number and position of the access points. If the WLAN coverage does not reach every area, the vehicles could lose contact and no longer communicate with the other vehicles and their control station.
Walls and contours
The laser scanner works at a certain height, as close as possible to the floor. This means that contours are only detected at this height. They should be as prominent as possible.Glass walls or highly reflective surfaces might not be detected. For these special cases, suitable measures must be planned to create contours visible to the laser scanner.
The floor should be as even as possible, but the vehicles can overcome slight ramps and edges.
The surface of the floor influences the braking distance and thus also the parameterization of the warning and protective fields of the safety laser scanner. Different floors can require different concepts for chassis and drive wheels.
Space requirement – width of aisles, oncoming traffic or roundabout, overtaking
In Germany, the required path widths are defined in the workplace regulation (Technical Rules for Workplaces ASR A1.8 – Traffic Routes) and depend on the vehicle width, which on the other hand depends on the size, shape and weight of the load.
The layout of the production area must be analysed with regard to the aisle widths and bottlenecks as well as escape routes, transfer points must be defined. It must be taken into account that the vehicles at transfer points stand longer and thus narrow the traffic routes. Already in the planning phase a rough planning in the layout is necessary, because the width of the paths, narrow places, one-way streets or one-AGV areas influence the possible driving speeds and thus also the capacity calculations.
To operate a vehicle fleet, a fleet control system is required. At InSystems this software is called AIC (AGV Interface Controller).
The AIC has an interface to the customer’s order management and translates the material requirement into transport orders for the vehicles. The AIC works like a taxi center and dispatches the transport orders to the optimal vehicle.
Criteria for assigning the order to a vehicle include the distance to the pick-up point, avoidance of empty runs and battery condition.
When planning the AIC, the interfaces to the existing production control system and the machines as well as all software functions (behaviour at time-out, fire alarm) must be planned.
The vehicle and the load handling attachment installed must be designed for the individual load. The transfer height plays a decisive role in determining how to design all elements (battery block, drives, vehicle control system, etc.) in the chassis. It must be ensured that the center of gravity in the loaded vehicle is as low as possible so that the vehicle cannot tip over during emergency braking.
It is better to pick up the load at the rear of the vehicle so that the it is safely guided by a stop during braking and cannot fall over the open side of the load pickup.
If the analysis of the production environment shows that the machines have different transfer heights, then it should be considered
- whether the vehicle can compensate the height with a lifting device (disadvantage: higher vehicle weight and higher energy consumption).
- whether load ports with a lifting device, which are mounted in front of the machine in-/outfeeds could be a solution.
- whether the transfer height can be brought to a uniform level by adjustments to the machinery.